Potassium cryolite for aluminum electrolysis industry and preparation method thereof

ABSTRACT

The invention provides a potassium cryolite for aluminum electrolysis industry, which has a molecular formula: mKF.AlF 3 , wherein m is from 1 to 1.5. The low-molecular-ratio potassium cryolite (mKF.AlF 3 , and m is from 1 to 1.5) provided by the invention is used for aluminum electrolysis industry, and can improve the dissolvability of aluminum oxide, thus reducing the temperature of electrolysis and the consumption of power, raising the efficiency of electrolysis and lowering the comprehensive production cost.

TECHNICAL FIELD OF THE INVENTION

The invention relates to potassium cryolite, more particularly to apotassium cryolite for aluminum electrolysis industry and a preparationmethod thereof.

BACKGROUND OF THE INVENTION

The traditional Hall-Heroult method has still been employed in aluminumelectrolysis industry so far and cryolite-aluminum oxide has served asthe basic system for electrolyte all the time, and in this system,sodium hexafluoroaluminate is usually used as cryolite. The temperatureof electrolysis in aluminum electrolysis industry is about 960° C.,which leads to high consumption of power, this is mainly because of highprimary crystal temperature of electrolyte, besides, in order to keepgood solubility of aluminum oxide, the degree of superheat at aparticular temperature needs to be kept.

The method for industrially preparing potassium fluoroaluminate(potassium cryolite) mainly is chemical synthesis method:fluoroaluminate is generated by the reaction between anhydroushydrofluoric acid and aluminum hydroxide, then reacted with potassiumhydroxide at high temperature, and finally filtered, dried, molten andcrushed to obtain potassium fluoroaluminate product; the potassiumfluoroaluminate, which is synthesized using such a method, has arelative molecular weight of 258.28, a molecular formula of mKF.AlF3(m=3.0) and a melting point of 560-580° C. The potassium cryoliteprepared using the current industrial synthesis methods generally has amolecular ratio m between 2.0 and 3.0, so it is difficult to preparelow-molecular-ratio potassium cryolite, which is pure and extremely lowin water content and has a molecular ratio m between 1.0 and 1.5.

Therefore, the shortcomings like high power consumption in electrolysisand unsatisfactory electrolyte exist in the prior art.

SUMMARY OF THE INVENTION

To solve the technical problems in the prior art, the inventor has madetremendous researches on electrolyte selection and preparation methodand has unexpectedly found that, the temperature of electrolysis inaluminum electrolysis industry can be reduced by usinglow-molecular-ratio potassium cryolite, which is prepared from the rawmaterial of potassium fluotitanate, potassium fluoborate or the mixturethereof according to a thermo-chemical synthesis method, as electrolytefor aluminum electrolysis system, therefore, the consumption of power isreduced and the comprehensive production cost is lowered.

The invention provides a potassium cryolite for aluminum electrolysisindustry, which has a chemical formula of mKF.AlF3, wherein m is from 1to 1.5.

By adopting the technical proposal above, the low-molecular-ratiopotassium cryolite (mKF.AlF3, and m is from 1 to 1.5) provided by theinvention is used for aluminum electrolysis industry, and can improvethe dissolvability of aluminum oxide, thus reducing the temperature ofelectrolysis and the consumption of power, raising the efficiency ofelectrolysis and lowering the comprehensive production cost.

As a further improvement of the invention, the m is 1, 1.2 or 1.5; themelting point of KF.AlF3 is about 555° C., the melting point of

$\frac{6}{5}{{KF} \cdot {AlF}_{3}}$

is about 556° C., and the melting point of

$\frac{3}{2}{{KF} \cdot {AlF}_{3}}$

is about 557° C.

The dissolubility of aluminum oxide in 3NaF.AlF3 molten liquid is about7 g/L, and the dissolubility of aluminum oxide in

${\frac{3}{2}{{KF} \cdot {AlF}_{3}}},$

KF.AlF3 and

$\frac{6}{5}{{KF} \cdot {AlF}_{3}}$

molten liquids is all within a range from 15 g/l to 20 g/l compared withsodium cryolite 3NaF.AlF3, so the dissolubility increased remarkably;the low-molecular-ratio potassium cryolites

${\frac{3}{2}{{KF} \cdot {AlF}_{3}}},$

KF.AlF3 and

$\frac{6}{5}{{KF} \cdot {AlF}_{3}}$

can reduce the temperature of electrolysis, lower the consumption ofpower and improve the efficiency of electrolysis when used for aluminumelectrolysis industry.

Correspondingly, the invention further provides a preparation method ofpotassium cryolite for aluminum electrolysis industry, comprising thefollowing steps:

A) aluminum is put in a reactor, which is heated up to 700-850° C. andthen added with potassium fluotitanate, potassium fluoborate or themixture thereof; and

B) after the mixture in the reactor is stirred for 4-6 hours, liquidmolten at the upper layer is sucked out to obtain potassium cryolite.

The preparation method provided by the invention has the advantages ofmoderate reaction conditions, easy control, simple process flow,complete reaction and good quality of product.

As a further improvement of the invention, the preparation method ofpotassium cryolite for aluminum electrolysis industry comprises thefollowing steps:

A) aluminum is put in a reactor that is a closed container, inert gas isfed into the reactor after evacuation, and the reactor is heated up to780-850° C. and then added with potassium fluotitanate; and

B) after the mixture in the reactor is stirred for 4-6 hours, liquidmolten at the upper layer is sucked out to obtain potassium cryolitehaving a chemical formula:

$\frac{3}{2}{{KF} \cdot {{AlF}_{3}.}}$

The chemical reaction formula involved is

${{\frac{3}{4}K_{2}{TiF}_{6}} + {Al}} = {{\frac{3}{4}{Ti}} + {\frac{3}{2}{{KF} \cdot {{AlF}_{3}.}}}}$

As a further improvement of the invention, the preparation method ofpotassium cryolite for aluminum electrolysis industry comprises thefollowing steps:

A) aluminum is put in a reactor that is a closed container, inert gas isfed into the reactor after evacuation, and the reactor is heated up to700-850° C. and then added with potassium fluoborate; and

B) after the mixture in the reactor is stirred for 4-6 hours, liquidmolten at the upper layer is sucked out to obtain potassium cryolitehaving a chemical formula: KF.AlF3. The chemical reaction formulainvolved is KBF4+Al=B+KF.AlF3.

As a further improvement of the invention, the preparation method ofpotassium cryolite for aluminum electrolysis industry comprises thefollowing steps:

A) aluminum is put in a reactor that is a closed container, inert gas isfed into the reactor after evacuation, and the reactor is heated up to700-850° C. and then added with the mixture of potassium fluoborate andpotassium fluotitanate based on a molar ratio of 2:1; and

B) after the mixture in the reactor is stirred for 4-6 hours, liquidmolten at the upper layer is sucked out to obtain potassium cryolitehaving a chemical formula:

$\frac{6}{5}{{KF} \cdot {{AlF}_{3}.}}$

The chemical reaction formula involved is

${{K_{2}{TiF}_{6}} + {2{KBF}_{4}} + {\frac{10}{3}{Al}}} = {{TiB}_{2} + {{\frac{10}{3}\left\lbrack {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right\rbrack}.}}$

As a further improvement of the invention, the preparation method ofpotassium cryolite for aluminum electrolysis industry comprises thefollowing steps:

A) aluminum is put in a reactor, which is heated up to 700-850° C. andthen added with the mixture of potassium fluoborate and potassiumfluotitanate based on a molar ratio of y:x; and

B) after the mixture in the reactor is stirred for 0.5-6 hours, liquidmolten at the upper layer is sucked out to obtain potassium cryolitehaving a chemical formula:

$\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {AlF}_{3}}$

The chemical reaction formula involved is

$\left. {{K_{2}{TiF}_{6}} + {KBF}_{4} + {Al}}\rightarrow{{{Al} \cdot {Ti} \cdot B} + {\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {{AlF}_{3}.}}}} \right.$

Compared with the prior art, the invention has the advantage that: thelow-molecular-ratio potassium cryolite (mKF.AlF3, and m is from 1 to1.5) provided by the invention, with a proper electric conductivity, isused for aluminum electrolysis industry, and can improve thedissolvability of aluminum oxide, thus reducing the temperature ofelectrolysis and the consumption of power, raising the efficiency ofelectrolysis and lowering the comprehensive production cost; thepreparation method of low-molecular-ratio potassium cryolite provided bythe invention is moderate in reaction conditions, easy in control,simple in process flow, complete in reaction and good in quality ofproduct.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Further detailed description is made to the invention with reference tothe embodiments below.

Embodiment 1

1 ton of aluminum is weighed and put in a reactor that is a closedcontainer, argon is fed into the reactor for the purpose of protectionafter evacuation, the reactor is heated up to 800° C. and then slowlyadded with dry potassium fluotitanate based on a reaction proportion,sponge titanium and potassium cryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right)$

are generated after the mixture in the reactor is rapidly stirred for 5hours, the cover of the reactor is opened and liquid-state potassiumcryolite molten at the upper layer is sucked out by a siphon pump.

The potassium cryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right)$

prepared is used for aluminum electrolysis industry, the electrolytesystem consists of potassium cryolite and aluminum oxide, thetemperature for electrolysis can be controlled within a working rangefrom 750° C. to 850° C. by using the electrolyte consisting of thepotassium cryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right)$

provided by the invention, and primary aluminum can be obtained by usinginert electrode material, carbon electrode material or mixed (co-use ofcarbon electrode material and inert electrode material) electrodematerial for electrolysis.

Embodiment 2

1 ton of aluminum is weighed and put in a reactor that is a closedcontainer, argon is fed into the reactor for the purpose of protectionafter evacuation, the reactor is heated up to 780° C. and then slowlyadded with dry potassium fluoborate based on a reaction proportion,boron and potassium cryolite (KF.AlF3) are generated after the mixturein the reactor is rapidly stirred for 5 hours, the cover of the reactoris opened and liquid-state potassium cryolite molten at the upper layeris sucked out by a siphon pump.

The potassium cryolite (KF.AlF3) prepared is used for aluminumelectrolysis industry, the electrolyte system consists of potassiumcryolite and aluminum oxide, the temperature for electrolysis can becontrolled within a working range from 750° C. to 850° C. by using theelectrolyte consisting of the potassium cryolite (KF.AlF3) provided bythe invention, and primary aluminum can be obtained by using inertelectrode material, carbon electrode material or mixed (co-use of carbonelectrode material and inert electrode material) electrode material forelectrolysis.

Embodiment 3

1 ton of aluminum is weighed and put in a reactor that is a closedcontainer, argon is fed into the reactor for the purpose of protectionafter evacuation, the reactor is heated up to 750° C. and then slowlyadded with dry mixture of potassium fluoborate and potassiumfluotitanate based on a reaction proportion, the molar ratio ofpotassium fluoborate to potassium fluotitanate is 2:1, titanium borideand potassium cryolite

$\left( {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right)$

are generated after the mixture in the reactor is rapidly stirred for 5hours, the cover of the reactor is opened and liquid-state potassiumcryolite molten at the upper layer is sucked out by a siphon pump.

The potassium cryolite

$\left( {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right)$

prepared is used for aluminum electrolysis industry, the electrolytesystem consists of potassium cryolite and aluminum oxide, thetemperature for electrolysis can be controlled within a working rangefrom 750° C. to 850° C. by using the electrolyte consisting of thepotassium cryolite

$\left( {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right)$

provided by the invention, and primary aluminum can be obtained by usinginert electrode material, carbon electrode material or mixed (co-use ofcarbon electrode material and inert electrode material) electrodematerial for electrolysis.

Embodiment 4

5 tons of aluminum is weighed and put in a reactor, the reactor isheated up to 750° C. and then slowly added with 2 tons of dry mixture ofpotassium fluoborate and potassium fluotitanate, the molar ratio ofpotassium fluoborate to potassium fluotitanate is 1:1,aluminum-titanium-boron alloy and potassium cryolite

$\left( {\frac{9}{7}{{KF} \cdot {AlF}_{3}}} \right)$

are generated due to excessive amount of aluminum after the mixture inthe reactor is rapidly stirred for 4 hours, the cover of the reactor isopened and liquid-state potassium cryolite molten at the upper layer issucked out by a siphon pump.

The potassium cryolite

$\left( {\frac{9}{7}{{KF} \cdot {AlF}_{3}}} \right)$

prepared is used for aluminum electrolysis industry, the electrolytesystem consists of potassium cryolite and aluminum oxide, thetemperature for electrolysis can be controlled within a working rangefrom 750° C. to 850° C. by using the electrolyte consisting of thepotassium cryolite

$\left( {\frac{9}{7}{{KF} \cdot {AlF}_{3}}} \right)$

provided by the invention, and primary aluminum can be obtained by usinginert electrode material, carbon electrode material or mixed (co-use ofcarbon electrode material and inert electrode material) electrodematerial for electrolysis.

The content discussed above is merely for further detailed descriptionof the invention with reference to the preferred embodiments, and itshall not be considered that the embodiments of the invention arelimited to the description only. Many simple deductions or substitutionscould be made without departing from the concept of the invention byordinary skilled in the art to which the invention pertains, and shallbe contemplated as being within the scope of the invention.

What is claimed is:
 1. A potassium cryolite for aluminum electrolysisindustry, characterized in that: the potassium cryolite has a molecularformula: mKF.AlF₃, wherein m is from 1 to 1.5.
 2. A preparation methodof potassium cryolite for aluminum electrolysis industry according toclaim 1, wherein the method comprises the following steps: A) aluminumis put in a reactor, which is heated up to 700-850° C. and then addedwith potassium fluotitanate, potassium fluoborate or the mixture thereof(sodium fluorotitanate, sodium fluoborate or the mixture thereof); andB) after the mixture in the reactor is stirred for 4-6 hours, liquidmolten at the upper layer is sucked out to obtain potassium cryolite. 3.The preparation method of potassium cryolite for aluminum electrolysisindustry according to claim 2, wherein the method comprises thefollowing steps: A) aluminum is put in a reactor that is a closedcontainer, inert gas is fed into the reactor after evacuation, and thereactor is heated up to 780-850° C. and then added with potassiumfluotitanate; and B) after the mixture in the reactor is stirred for 4-6hours, liquid molten at the upper layer is sucked out to obtainpotassium cryolite having a chemical formula:$\frac{3}{2}{{KF} \cdot {{AlF}_{3}.}}$
 4. The preparation method ofpotassium cryolite for aluminum electrolysis industry according to claim2, wherein the method comprises the following steps: A) aluminum is putin a reactor that is a closed container, inert gas is fed into thereactor after evacuation, and the reactor is heated up to 700-850° C.and then added with potassium fluoborate; and B) after the mixture inthe reactor is stirred for 4-6 hours, liquid molten at the upper layeris sucked out to obtain potassium cryolite having a chemical formula:KF.AlF₃.
 5. The preparation method of potassium cryolite for aluminumelectrolysis industry according to claim 2, wherein the method comprisesthe following steps: A) aluminum is put in a reactor that is a closedcontainer, inert gas is fed into the reactor after evacuation, and thereactor is heated up to 700-850° C. and then added with the mixture ofpotassium fluoborate and potassium fluotitanate based on a molar ratioof 2:1; and B) after the mixture in the reactor is stirred for 4-6hours, liquid molten at the upper layer is sucked out to obtainpotassium cryolite having a chemical formula:$\frac{6}{5}{{KF} \cdot {{AlF}_{3}.}}$
 6. The preparation method ofpotassium cryolite for aluminum electrolysis industry according to claim2, wherein the method comprises the following steps: A) aluminum is putin a reactor, which is heated up to 700-850° C. and then added with themixture of potassium fluoborate and potassium fluotitanate based on amolar ratio of y:x; and B) after the mixture in the reactor is stirredfor 0.5-6 hours, liquid molten at the upper layer is sucked out toobtain potassium cryolite having a chemical formula:$\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {{AlF}_{3}.}}$
 7. Thepotassium cryolite for aluminum electrolysis industry according to claim1, wherein the m is 1, 1.2 or 1.5.
 8. A preparation method of potassiumcryolite for aluminum electrolysis industry according to claim 7,wherein the method comprises the following steps: A) aluminum is put ina reactor, which is heated up to 700-850° C. and then added withpotassium fluotitanate, potassium fluoborate or the mixture thereof(sodium fluorotitanate, sodium fluoborate or the mixture thereof); andB) after the mixture in the reactor is stirred for 4-6 hours, liquidmolten at the upper layer is sucked out to obtain potassium cryolite. 9.The preparation method of potassium cryolite for aluminum electrolysisindustry according to claim 8, wherein the method comprises thefollowing steps: A) aluminum is put in a reactor that is a closedcontainer, inert gas is fed into the reactor after evacuation, and thereactor is heated up to 780-850° C. and then added with potassiumfluotitanate; and B) after the mixture in the reactor is stirred for 4-6hours, liquid molten at the upper layer is sucked out to obtainpotassium cryolite having a chemical formula:$\frac{3}{2}{{KF} \cdot {{AlF}_{3}.}}$
 10. The preparation method ofpotassium cryolite for aluminum electrolysis industry according to claim8, wherein the method comprises the following steps: A) aluminum is putin a reactor that is a closed container, inert gas is fed into thereactor after evacuation, and the reactor is heated up to 700-850° C.and then added with potassium fluoborate; and B) after the mixture inthe reactor is stirred for 4-6 hours, liquid molten at the upper layeris sucked out to obtain potassium cryolite having a chemical formula:KF.AI F₃.
 11. The preparation method of potassium cryolite for aluminumelectrolysis industry according to claim 8, wherein the method comprisesthe following steps: A) aluminum is put in a reactor that is a closedcontainer, inert gas is fed into the reactor after evacuation, and thereactor is heated up to 700-850° C. and then added with the mixture ofpotassium fluoborate and potassium fluotitanate based on a molar ratioof 2:1; and B) after the mixture in the reactor is stirred for 4-6hours, liquid molten at the upper layer is sucked out to obtainpotassium cryolite having a chemical formula:$\frac{6}{5}{{KF} \cdot {{AlF}_{3}.}}$
 12. The preparation method ofpotassium cryolite for aluminum electrolysis industry according to claim8, wherein the method comprises the following steps: A) aluminum is putin a reactor, which is heated up to 700-850° C. and then added with themixture of potassium fluoborate and potassium fluotitanate based on amolar ratio of y:x; and B) after the mixture in the reactor is stirredfor 0.5-6 hours, liquid molten at the upper layer is sucked out toobtain potassium cryolite having a chemical formula:$\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {{AlF}_{3}.}}$